video: tegra: nvmap: Fix heap compactor under OOM

The carveout heap compactor wasn't really prepared for running out of
memory, and would trigger a BUG_ON instead.

Bug 1024556

Change-Id: I661942d787a1fe59ab35e035d124e4cfaeed69b3
Signed-off-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Reviewed-on: http://git-master/r/131889
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Krishna Reddy <vdumpa@nvidia.com>